ERA22LA117

A loss of engine power due to bearing failures in the turbine section resulting from a fatigue fracture of the oil supply line that fed oil to the Nos. 6 and 7 bearings. The oil line failure led to rapid deterioration of the bearings from oil starvation. Contributing to the power loss was the installation of a No. 8 stationary seal with an undersized outside diameter, which resulted in a reduction of support for the turbine section rotating components and resulted in high vibration loads in the engine, which ultimately caused the oil supply line to fatigue and fail. Also contributing was the improper or inadequate inspections of the No. 8 stationary seal by maintenance personnel.

The pilot reported that he had heard a whistling sound with the engine while conducting powerline operations and landed out of precaution. He examined the engine and consulted with maintenance personnel, but no mechanical anomalies were found. The helicopter was grounded for the workday; however, at the end of the day, a decision was made to try and return the helicopter back to its base of operations. During the flight, the engine chip light illuminated followed by the smell of engine oil and a grinding noise. The pilot attempted a precautionary landing to a field, but smoke filled the cabin, reducing his visibility while in the landing flare. The pilot attempted to slow the rate of descent and impacted the ground in a near-level attitude. During the ground run the front portion of the skids dug into the ground, causing the helicopter to pitch forward. The pilot applied aft cyclic to keep the helicopter level. During the landing sequence the main rotor blades struck the tail boom, which resulted in the horizontal and vertical stabilizers and the tail rotor assembly separating from the helicopter. Postaccident examination of airframe and engine revealed residual oil on the interior and exterior of the engine access doors and on the interior of the engine compartment. The oil supply line that feeds oil to the Nos. 6 and 7 bearings was fractured along with its support bracket. Numerous other components including the gearbox housing, N1 coupling, gas producer (GP) turbine support assembly (which included a sump nut, retaining ring and plate, the No. 8 oil supply jet, and a fractured shear pin), No. 8 bearing, No. 8 rotating seal, No. 8 stationary seal, and the outer combustion chamber, had also fractured and/or sustained high heat damage. Evidence of fretting damage was also observed on multiple components. The National Transportation Safety Board (NTSB) Materials Laboratory analyzed these components and determined that the engine most likely failed due to bearing failures in the turbine section resulting from the high-cycle fatigue fracture of the oil supply line that fed oil to the Nos. 6 and 7 bearings. The oil line failure led to rapid deterioration of the bearings from oil starvation, resulting in misalignment of rotating components and interference with stationary components within the engine, producing the grinding noise noted by the pilot. The oil line fracture was also likely associated with the smell of oil followed by the smoke in the cockpit reported by the pilot. Further examination of the engine revealed that the engine failure likely started with the No. 8 bearing stationary seal. The outside diameter of the seal was undersized, so it did not have the specified interference fit with the GP support hub. The improper fit likely led to insufficient support for the No. 8 bearing and excessive flexing of the No. 8 stationary seal cup wall. As a result, the stationary seal developed fatigue cracks and eventually fractured. The lack of interference fit with the No. 8 stationary seal likely affected the effectiveness of the seal between the stationary and rotating seals, which could have allowed oil to escape forward past the seal and into the gas path. The fracture of the No. 8 stationary seal reduced the support for the rotating turbine components at the No. 8 bearing, which likely led to increased vibrations in the engine. Fractures in the outer combustion chamber, oil line clamp, and gearbox case housing all had indications of high-cycle fatigue fracture from vibration loading. These failures likely resulted from excessive vibrations associated with the reduction in support for the turbine section rotating components. The fractured oil line support clamp failed followed by the oil supply line. According to overhaul records, the turbine section was last overhauled in July 2020, when the GP turbine wheels were replaced due to service time limits. A review of the engine manufacturer’s overhaul maintenance manual (OHM) revealed the condition of the No. 8 stationary seal should have been inspected; however, there was no indication in the overhaul records that the No. 8 stationary seal had been inspected, removed, or replaced. The turbine module was removed by the operator a few months after the overhaul due to a N2 lockup and sent to a repair facility. The repair facility ended up removing the GP support and sending the unit to another facility where the 4th stage wheel was replaced. According to the engine manufacturer, the repair facility should have been following the same OHM inspection criteria that included inspection of the No. 8 stationary seal. According to the repair facility, they had no record that the No. 8 stationary was repaired/replaced at the time the GP support/4th stage wheel was replaced.

HISTORY OF FLIGHT On February 9, 2022, at 1628 eastern standard time, a McDonald Douglas MD-369D helicopter, N9159F, was substantially damaged when it was involved in an accident near Bel Air, Maryland. The commercial pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight. The pilot stated that while conducting powerline inspection work earlier in the day, line personnel reported that the helicopter was making a strange "whistle" noise. The pilot inspected the helicopter and no anomalies were noted or observed. The pilot continued with normal operations, but the noise continued and one of the operator’s superintendents took a video, where a “whistle”-like sound could be heard. The pilot landed and ceased all human external cargo operations. He then reviewed the video, re-examined the helicopter, and spoke with company maintenance personnel. Though no obvious mechanical issues were observed, the pilot “parked” the helicopter for the remainder of the workday. At the end of the workday, the pilot again inspected the aircraft and found no mechanical reason not to reposition the helicopter back to its normal base of operations. He and another company helicopter departed as a flight of two. Several minutes into the flight, the pilot said the ENGINE CHIP light illuminated. He told the other pilot that even though the engine seemed to be operating normally, he would need to land as soon as practicable. Shortly after, the engine began to make a “grinding” noise along with an odor of engine oil, which eventually became smoke in the aft section of the passenger compartment. With the presence of smoke and the potential for an inflight fire, the pilot initiated an emergency descent-to-land to a suitable landing area. During the descent the engine noise and smoke in the aft section of the cabin intensified and began moving to the forward section of the cockpit. Descending through the landing flare, as the pilot leveled the helicopter to land, the engine stopped producing power and smoke in the cockpit reduced his visual reference to the ground. The pilot attempted to slow the rate of descent and impacted the ground in a near-level attitude. During the ground run the front portion of the skids dug into the ground, causing the helicopter to pitch forward. The pilot applied aft cyclic to keep the helicopter level. During the landing sequence, the main rotor blades struck the tail boom, which resulted in the horizontal and vertical stabilizers and the tail rotor assembly separating from the helicopter. Residual oil was observed on the interior and exterior surfaces of the engine access doors and on the interior of the engine compartment. AIRFRAME AND ENGINE EXAMINATION The helicopter was recovered by the operator and taken to their facility in Gettysburg, Pennsylvania. Before the engine was removed, an external examination of the lubrication system between the airframe and the engine was conducted. Neither the aircraft-mounted oil reservoir nor the oil cooler were damaged, and no residual oil was noted within the cabin area (where the cooler and reservoir were located). The engine was then removed from the airframe and shipped to Keystone Turbine Services, Coatesville, Pennsylvania, where a full engine examination was conducted under the supervision of the NTSB. Examination of the engine revealed that each of the external oil lines were secure except for the oil line which supplies pressure oil to the turbine sumps. The line connected to a horizontal fire shield and a T-fitting near the Nos. 6 and 7 bearing sump. The line was fractured at the horizontal fire shield and misaligned. Also, the clamp that secured the line to the turbine module was fractured. The scavenge oil filter was clean and full of oil. A small amount of ferrous debris was observed on the filter. The pending bypass button was not extended. Both the upper and lower engine magnetic chip detector plugs were removed, and ferrous debris was observed on both. The gas producer turbine rotor (N1) was seized but the power turbine (N2) turned and was connected to the powertrain. No damage was noted on the first stage compressor blades or compressor inlet. The fourth stage turbine wheel was normal in appearance when viewed from the exhaust collector. The accessory gearbox was intact except for one fractured compressor mount pad. As the compressor was removed, the mount was found liberated from the gearbox housing. The gearbox housing and cover were split apart, and all the internal gears and bearings were intact, except for the No. 2 ½ bearing. The No. 2 ½ bearing was missing 6 rollers, consistent with having fallen out during the compressor removal, and were recovered from the bottom of the engine stand. The rollers were undamaged. The compressor bore on the gearbox cover displayed a wear step between the 1:00 to 12:00 position. The combustion section was not damaged. The inner surface of the outer combustion case barrel was missing material and was cracked. Several pieces of barrel material were recovered from the turbine inlet. The combustion liner was covered in carbon, but no mechanical damage was observed. Several areas within the turbine module displayed evidence of oil starvation: • No. 8 bearing cavity • No. 8 bearing sump oil scavenge line (was also clogged with debris/dry residue) • No. 8 bearing sump oil supply tubes • No. 6/7 bearing cavity • No. 6/7 bearing scavenge orifice • No. 6/7 bearing oil supply tube • No. 6/7 bearing external sump can The No. 6 bearing rollers were disintegrated, which precluded inspection of the No. 7 bearing. The No. 8 bearing balls were also disintegrated and the remaining components (inner race, outer race, and separator) were removed. The bearing components displayed thermal signatures consistent with overtemperature operation. The gas producer (GP) rotor tie bolt nut was loose but remained in position within the locking feature; the GP rotor was intact. The trailing edge blade tips of the first stage turbine wheel displayed mechanical damage and the upstream face of the wheel was circumferentially gouged and smeared. The energy absorbing ring location tabs and corresponding slots in the GP support displayed heavy wear. The No. 5 bearing turned freely and smoothly and displayed some brown discoloration. The N2 rotor was removed and all airfoils were intact. The upstream face of the third stage turbine wheel was coated in a gray carbon-like substance. The N2 shaft was intact with some discoloration and carbon noted on the exterior surface. The N1 shaft was intact and exhibited some slight bulging at the end of the shaft. The engine compressor turbine assembly, gearbox housing, oil pump, oil supply line with fittings and clamp assembly, N1 coupling, GP turbine assembly, GP turbine support (which included a sump nut, retaining ring and plate, and the No. 8 oil supply jet), No. 8 bearing, No. 8 rotating and stationary seal, No. 8 bearing spanner nut, and the outer combustion chamber, were sent to the NTSB Materials Laboratory for examination. Examination of these components by the NTSB Materials Laboratory determined numerous instances of high heat damage and fracture due to high-cycle fatigue. The gearbox housing support lug located on the upper left side of the gearbox exhibited fracture surfaces consistent with high-cycle fatigue. The oil pump, which was installed on the interior of the gearbox, was intact. There were no fretting contact marks noted on the pump mating to blended area on gearbox housing. Pitting consistent with cavitation damage were noted on gear teeth in the pressure body. The oil supply line that was found fractured during the engine exam, and its associated support clamp, also exhibited fracture surfaces consistent with high-cycle fatigue. This oil line supplied oil to the Nos. 6 and 7 bearings in the turbine section. The